antman89iroc

Ok this is a trick question. I don't run an automatic. But the same engine parameters which affect the "stall" speed (of an ideal torque converter-automatic) should apply to any engine.
I'm running a 414sbc with 236/242 600 lift cam cut on 112 centerline. From what I have been able to determine a ~25-2800 stall would be about right for a primarily street driven vehicle. Now, if we operate that same engine under that RPM, how does it run? What are the symptoms?
I have what I describe as a surge or hesitation around 15-1800 RPM at light throttle, say 45 map. With a stick shift this very annoying. It reminds me of how a drag car sounds driving through the pits- varump-varump-varump.
What I am trying to determine is how to get rid of this. I have tried almost every combination of timing and fuel. I am at the point where I think the only way to change it is to change camshafts. This isn't a radical cam really, I guess I am just wanting it to have better manors at the lower rpms and at light throttle. The variable is how much duration, or LSA, or combination of both will need to change? I am sorry if this seems a little out of this section's normal topic but I have searched all over the net and read a lot of "cam" and "stall" articles and they don't really answer the question. I feel that someone with a lot of experience building or running cammed engines and trying different stall speeds will know exactly what I am describing and how to correct it without losing a lot of top end. Currently the engine/cam makes ~420 WHP at 6000 and has a very flat power curve.
I believe the HSR intake (single plane basically) along with the cam duration and overlap are both affecting the condition in a negative way.

RBob

iTrader: (1)

Try running the ECM in open loop and see how it is.

RBob.

antman89iroc

I've been tuning/running in open loop for about a year. I still have the same problem. It does get somewhat better with more fuel. Weird thing is the OL tune gets lean as the temp comes up to ~205f. When I tune rich at that temp it runs very rich when cooler. Even with 195 stat it will pull down to ~185 when moving. I've tried a lot of different combinations of CT/IAT blend too.
I recently went back to closed loop and it seems better although still a problem. EBL adds fuel when it gets warm-er.
But what about the bucking/surging with this cam at lower rpms? Do you really think I have a tune problem. If I can get this addressed it runs well. I think I went with a little too much cam if I am honest. Not the first time lol.

Night rider327

iTrader: (13)

I can't comment on the EFI side of things as I'm a carb guy, but I don't think it's your cam size. A 236/242, 112 lsa cam in a big 414 cid engine is on the milder side really. What kind of idle vac are you pulling? I'm guessing about 13-14" hg.

The cam, with your engine size and gearing should not cause any bucking or surging over about 1,100 rpm.

As far auto cars, driving under stall speed. The only thing you notice really is more converter slip, and a little laziness to throttle response.

I run a 10.5, 355 with 200cc heads, 249/252, 106 lsa cam. I have to idle at 1,000 rpm, and I only pull 3-4" hg vac at that speed. The engine smooths out with no bucking at as low as 1,500 rpm. I now run a 3500 stall, 3.89 gear, but did drive the car for a couple of years with a 2800 stall and 3.42 gear.

antman89iroc

As far as EFI vs carb there are a few things which I think are a factor in "stall" speed and tuning. I would think efi would be able to run better at lower rpm since it doesn't rely on the flow signal for fuel flow. In my case the LSA is wider so there is less overlap and it seems both of those would improve performance at rpms lower than stall.

As far as idle vacuum you are right on target. My gauge shows around 13" when idling at 900rpm and AC off. A little less vacuum and 950rpm with AC on.

The engine pulls good from ~1000 and up when under a little load. It pulls at 1500 pretty smoothly but as the speed increases to normal in-town speeds of say 45mph and the engine is up to around 1800rpm (5th gear) then the load decreases and that is where it bucks and surges. Add a little throttle and it smoothly pulls. It's when I am at cruise load levels that it acts up. I think the combination of vacuum in the plenum and cam overlap is causing some weird things to happen to the intake charge and the fuel.

So, for example, if I still had the 700 trans with a lock up TQ, how would you expect a motor with a cam like mine to run at the lower engine speeds?

RBob

iTrader: (1)

Antman, just had a thought, is it possible that it is drive-line windup? This is a very real problem that even GM has SAE papers on. I experience it with one stick shift car I drive from time-to-time.

I've learned to de-clutch immediately to stop it. Then work with the clutch and/or throttle until the vehicle it out of that area of concern.

I think that the next item has already been mentioned, but in case it hasn't: check a data log to see if the ECM is switching between single fire (S/F) mode and double fire mode. Double fire in the normal mode with S/F being indicated in the WUD.

Unless the injector compensations are dead on this mode switching can cause this type of issue.

RBob.

redneckjoe

this is just my opinion; i think its a slightly large cam to be street driving/lugging around in 5th gear. it probably just wants to go! some more rear gear may help?